2000 Mercury Marquis STARTER

That cannot happen unless the starter solenoid contacts are closed and they can only be closed if the solenoid is being energized by the ignition starter switch or if arcing between the contacts have welded them closed. It is very simple to determine if the problem is in the switch or the solenoid. Your vehicle will have a neutral safety switch in the circuit between the ignition starter switch and the starter solenoid.Put the shift lever in either drive or reverse to open the neutral safety switch circuit and assure that the ignition starter switch cannot energize the starter solenoid and then connect your battery. If the starter engages with the ignition starter switch off then the problem is in the solenoid (arcing between the contacts have welded them closed) but if starter does not engage (because the neutral safety switch is open and the circuit from the ignition starter switch and the starter solenoid is not complete) then the problem is in the ignition starter switch.